Detecting path/trajectory turns in tracking data
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I have some 2D cartesian position data (x,y,time) for a moving object, I would love the extract all of the points where the path/trajectory of this object changed in a particular way. For instance, I would like to know everywhere the object made a turn greater than 90 degrees in less than 2 seconds.
I tried extracting the object's instantaneous heading and then looking for places where this heading changes by an angle greater than a certain cutoff, however, this seems to be missing a lot of turns. I was wondering if there was some clever Matlab way to do what I want?
EDIT: I have attached some sample data, this is one file of many which I have to process. The data are arranged X,Y and time. The data are sampled at 10Hz I believe.
Star Strider el 12 de Feb. de 2015
Here is one possibility, using filter:
d = load('Right Grievous SampleData.mat');
x = d.x(:,1);
y = d.x(:,2);
t = d.x(:,3);
dx = gradient(x, 0.1);
dy = gradient(y, 0.1);
dt = gradient(t);
hdc = pi/2; % Critical Heading Change
hdg = atan2(dy,dx); % Heading
sec = 2; % ‘Look Ahead’ Time (sec)
dhdg = filter([1 zeros(1,sec*10-2) 1], 2, hdg);
hdgidx = find(abs(dhdg) >= hdc); % Find ‘dhdg’ >= pi/2
plot3(x, y, t)
plot3(x(hdgidx), y(hdgidx), t(hdgidx), '.r')
It uses filter to look at two headings separated by the requisite number of seconds, calculates the differences, and plots them as red dots on the trajectory. I’m not sure what you want, so I encourage you to experiment with it until you get the result you want. You might also consider subtracting a constant from ‘hdgidx’ to change the red dot position from the 20th position to another. For instance, to put a dot at the midpoint instead of the end of the look-forward time, consider subtracting 10, or fix(10*sec/2).